We propose to investigate and determine the effect of a novel microRNA (miRNA)-mediated pathway in drug disposition and drug resistance. Elevated expression of drug transporters such as ABCB1 (MDR1) contributes to drug resistance mediated by active drug export. Expression of these transporters is transcriptionally controlled by two major xenobiotic receptors, constitutive androstane receptor (CAR) and pregnane X receptor (PXR), both of which function as heterodimers with retinoid X receptor (RXR). Whereas PXR is ligand- activated, CAR is constitutively active in the absence of ligand, and its expression leads to that of its transcriptional targets. Recent studies have provided evidence of miRNA-mediated drug resistance. Our preliminary studies found that miR-137 is down-regulated in doxorubicin-resistant cells, while CAR and MDR1 are up-regulated. We found that miR-137 and CAR form an unusual negative feedback loop wherein miR-137 directly reduces CAR levels, and CAR negatively regulates miR-137 expression. We also found that CAR inverse agonists reduce MDR1 and increase miR-137 levels, and have developed a novel CAR inverse agonist (LTC27) that is potent and selective for CAR. Importantly, ectopically expressed miR-137 reduces CAR and MDR1, and re-sensitizes doxorubicin-resistant cells to doxorubicin both in vitro and in vivo; suggesting that drug resistance can be reversed. However synthetic miRNAs have limited bioavailability because of their low stability and cellular uptake. By screening, we identified vorinostat (or SAHA) that induces miR-137 expression and reduce MDR1 level. SAHA, a drug approved by FDA, is an inhibitor of histone deacetylases (HDACs); HDAC inhibition can disrupt the transcriptionally repressive CAR-HDAC1 complex. Our preliminary data lead us to hypothesize that increasing miR-137 levels or inhibiting CAR will reduce MDR1 in drug-resistant cells, thereby increasing intracellular drug accumulation and re-sensitizing the cells; that HDAC inhibitors induce miR-137 expression, at least partly by relieving the repressive effect of CAR, and can reverse drug resistance. To test these hypotheses, we will determine (1) the extent to which doxorubicin resistance is reversed by ectopic expression of miR-137; (2) the extent to which doxorubicin resistance is reversed by CAR inverse agonists; and (3) the mechanism by which miR-137 expression is induced, and the extent to which drug resistance is reversed by HDAC inhibitors, in our unique doxorubicin- resistant models. We will also examine the HDAC1-CAR-miR-137 promoter complex and determine the extent to which the complex is disrupted by HDAC inhibitors. These studies will illustrate how drug resistance is acquired and identify potential treatments to reverse resistance.